Cermet is a term used to describe a monolithic material composed of a hard component and a binder component. The hard component comprises a nonmetallic compound or a metalloid. The hard component may or may not be interconnected in two or three dimensions. The binder component comprises a metal or alloy and is generally interconnected in three dimensions. The binder component cements the hard component together to form the monolithic material. Each monolithic cermet's properties are derived from the interplay of the characteristics of the hard component and the characteristics of the binder component. For example, if the hard component or the binder component exhibits ferromagnetic characteristics so might the monolithic cermet.
A cermet family may be defined as a monolithic cermet consisting of a specified hard component combined with a specified binder component. Tungsten carbide cemented together by a cobalt alloy is an example of a family (WC-Co family, a cemented carbide). The properties of a cermet family may be tailored, for example, by adjusting an amount, a characteristic feature, or an amount and a characteristic feature of each component separately or together. However, an improvement of one material property invariably decreases another. When, for example, in the WC-Co family as resistance to wear is improved, the resistance to breakage generally decreases. Thus, in the design of monolithic cemented carbides there is a never ending cycle that includes the improvement of one material property at the expense of another.
Despite this, monolithic cemented carbides are used in equipment subject to aggressive wear, impact, or both. However, rather than build the entire equipment from monolithic cemented carbides, only selected portions of the equipment comprise the monolithic cemented carbide. These portions experience the aggressive wear, impact, or both. In some equipment the cemented carbide portion has a specified profile that should be sustained to maintain the maximum efficiency of the equipment. As the specified profile changes, the equipment's efficiency decreases. If the equipment is used for cutting a work piece, the amount removed from the work piece decreases as the profile of the cemented carbide deviates from the specified profile.
For example, as the specified dome-shaped profiles of cemented carbide compacts used in conjunction with a percussive bit change, once optimally shaped cemented carbide compacts transform into flats by tangentially wearing away the domes. As flats gradually develop on the gage row, the cut hole diameter decreases. During the transformation from domes to flats, power supplied by a motor driving the percussive bit may be increased thereby increasing the rate of wear. One solution to the loss of a specified profile includes removing the equipment from use and reprofiling the cemented carbide--this is costly due to the time required to withdrawal the percussive bit from a hole (up to several hundred meters (feet) deep) and the loss of productive use of the equipment during reprofiling. Another solution involves scrapping the used cemented carbide portion and inserting a new cemented carbide--this too is costly, again, due to the time required to withdrawal the percussive bit from a hole (up to several hundred meters (feet) deep) and due to the loss of productive use of the equipment during refitting and the scrapped cemented carbide. Additional cost associated with this latter solution include reboring the undersized sections of the hole that developed as the domed cemented carbide transformed to flats. If these cemented carbides could be made to sustain their specified profiles for a longer time, for example, by increasing the wear resistance, economic and technical gains would result.
A solution to the endless cycle of adjusting one property of a monolithic cermet at the expense of another is to combine several monolithic cermets to form a multiple-region cermet article. The resources (i.e., both time and money) of many individuals and companies throughout the world have been directed to the development of multiple-region cemented carbide articles. The amount of resources directed to the development effort is demonstrated by the number of publications, US and foreign patents, and foreign patent publications on the subject. Some of the many US and foreign patents, and foreign patent publications include: U.S. Pat. Nos. 2,888,247; 3,909,895; 4,194,790; 4,359,355; 4,427,098; 4,722,405; 4,743,515; 4,820,482; 4,854,405; 5,074,623; 5,333,520; and 5,335,738, and foreign patent publication nos. DE-A-3 519 101; GB-A 806 406; EPA-O 111 600; DE-A-3 005 684; DE-A-3 519 738; FR-A-2 343 885; GB-A-1 115 908; GB-A-2 017 153; and EP-A-0 542 704. Despite the amount of resources dedicated, no satisfactory multipurpose multiple-region cemented carbide article is commercially available nor for that matter, currently exists. Further, there is no satisfactory methods for making multiple-region cemented carbide articles. Furthermore, there are no satisfactory methods for making multiple-region cemented carbide articles that further exhibit improved wear resistance.
Some resources have been expended for "thought experiments" and merely present wishes--in that they fail to teach the methods of making such multiple-region cemented carbide articles.
Other resources have been spent developing complicated methods. Some methods included the pre-engineering of starting ingredients, green body geometry or both. For example, the starting ingredients used to make a multiple-region cemented carbide article are independently formed as distinct green bodies. Sometimes, the independently formed green bodies are also independently sintered and, sometimes after grinding, assembled, for example, by soldering, brazing or shrink fitting to form a multiple-region cemented carbide article. Other times, independently formed green bodies are assembled and then sintered. The different combinations of the same ingredients that comprise the independently formed green bodies respond to sintering differently. Each combination of ingredients shrinks uniquely. Each combination of ingredients responds uniquely to a sintering temperature, time, atmosphere, or any combination of the preceding. Only the complex pre-engineering of forming dies and, thus, green body dimensions allows assembly followed by sintering. To allow the pre-engineering, an extensive data base containing the ingredients response to different temperatures, times, atmospheres, or any combination of the preceding is required. The building and maintaining of such a data base are cost prohibitive. To avoid those costs, elaborate process control equipment might be used. This too is expensive. Further, when using elaborate process control equipment, minor deviations from prescribed processing parameters rather than yielding useful multiple-region cemented carbide articles--yield scrap.
Still other resources have been expended on laborious methods for forming multiple-region cemented carbide articles. For example, substoichiometric monolithic cemented carbide articles are initially sintered. Their compositions are deficient with respect to carbon and thus the cemented carbides contain eta-phase. The monolithic cemented carbide articles are then subjected to a carburizing environment that reacts to eliminate the eta-phase from a periphery of each article. These methods, in addition to the pre-engineering of the ingredients, require intermediate processing steps and carburizing equipment. Furthermore, the resultant multiple-region cemented carbide articles offer only minimal benefits since once the carburized peripheral region wears away, their usefulness ceases.
For the foregoing reasons, there exists a need for multiple-region cermet articles and cemented carbide articles that can be inexpensively manufactured. Further, there exists a need for multiple-region cermet articles and cemented carbide articles that further exhibit superior wear resistance and can be inexpensively manufactured.